The present invention relates to ultrasound transducers. In particular, ultrasound transducers for electrical communication with an imaging system are provided.
Conventional ultrasound transducer arrays operate in a longitudinal extensional or k33 resonant mode. Each element of the array has an electrode on the top surface of the element and another on the bottom surface of the element. The element is poled orthogonal to the electrodes or in a direction extending between the electrodes. In response to a potential difference applied across the electrodes, vibration is generated on the same orientation as the poling. Acoustic energy propagates along a direction extending from the face of the element covered by one of the electrodes.
Due to the size constraints of elements within a multi-dimensional transducer array, multi-layered piezoelectric ceramics have been suggested to provide a better impedance match with a cable and/or the imaging system electronics. Layers of piezoelectric ceramics are stacked along the same dimension as the poling, along the vibration or thickness dimension. Alternating layers of electrodes are electrically connected in parallel, providing a capacitance proportional to the square of the number of layers. However, making multiple connections on these elements is difficult. Vias for forming the connections have been proposed, but this method is difficult and costly. Vias also reduce the active area for transduction. Where patterning and partial dicing are used, undiced ceramics may result in generation of undesirable acoustic modes. Using electrodes on the sides of the small multi-dimensional elements for k33 mode operation may result in poor performance due to undesired contributions of the electric field transverse to the displacement direction. Methods where hundreds or thousands of individual multi-layer piezoelectric actuator posts are created, wire bonded and re-assembled into an array can be difficult and costly.
Small single layer elements of a multi-dimensional ultrasonic array may have a very low capacitance when electrically connected. For example, a 250×250×300 micrometer single layer piezoelectric element for operation at 5 megahertz has a capacitance of about 2 picoFarads (pF) in a k33 resonant mode. Such capacitance may not effectively drive a cable electrical load of 50 to 100 pF without impedance matching. Impedance matching at the element adds undesired size to arrays, such as arrays meant for use within a patient, and may degrade the signal to noise ratio.
A composite PZT operating in k31 mode has been proposed for matching electrical impedance. A dicing kerf and conductive filler, such as silver epoxy, are used as electrodes. However, conductive epoxy may result in strong acoustical cross coupling between elements. Additionally, using a kerf as an electrode substantially reduces the active piezoelectric material, reducing efficiency of the device.